You really need to keep all your sub-targetting within the castsequence if you wish to return to the old target afterwards.
That's only neccesary if you use targetlasttarget. Which you can just drop anyhow. It's not neccesary. Everytime you press the button, it would get the target anew anyhow. No need to remember targets.

/targetlasttarget is actually buggy, but that's an entirely different subject.

I use /targetlastenemy and /cleartarget in my ftl macro's to keep my alt targets empty so it looks naic and to be able to target enemies even when blinded or otherwise incapacitated.